1. Field of the Invention
The present invention relates generally to high temperature steam generators for use in recovering crude oil of low specific gravity. More particularly, the present invention relates to enhanced steam generators using multiple heating stages to produce superheated steam.
2. Description of the Related Art
A variety of steam heaters and associated steam injection techniques have been proposed for recovering heavy crude oil deposits. It is well know in the art to inject high temperature steam within wells to decrease the viscosity of heavy crude oils, facilitating subsequent pumping and recovery. Injected steam warms the well bore, heating the piping, the casings and the environment. A recognized difficulty in the art relates to the generation of superheated steam at proper temperatures and volume. Injected steam must not only be of sufficient temperature and pressure to properly liquefy targeted crude oil within the well, but a sufficient volume of such steam is required during the injection process for success. In general, large volume demands mitigate against the successful operational maintenance of the requisite pressure and temperature of the applied steam.
Previously it has been known in the art to provide a steam heater with an internal tank positioned coaxially disposed within an outer shroud. It is known to use electric heating elements surrounded by lead disposed between the tank and the electrodes. As the lead melts from the heating elements, heat is transferred by the molten metal disposed about the steam vessel. This basic construction is shown in Mexican patent No. 97201, issued November 1968. However, with the latter device, steam output temperatures vary widely. Liquid levels within the input tank would vary constantly, resulting in irregular vaporization. Temperature fluctuation between Four Hundred to Sixteen Hundred Degrees F. were experienced, resulting in the inadvertent stopping of crude oil pumps in response to build-up of improperly heated steam.
Multiple stage steam generators for enhancing crude oil recovery are known in the art. U.S. Pat. No. 4,408,116 issued to Turner on Oct. 4, 1983 discloses a superheated steam generator with dual heating stages. The first stage comprises a plurality of radially spaced-apart heaters that surround an encircled, second stage heater. A primary manifold system supplies water to each of the first stage heaters via elongated tubes extending longitudinally interiorly of the first stage heater tanks. A rigid, tubular sheath coaxially surrounds and protects each of the last mentioned tubes, and defines a steam output passageway between the sheath and the mouth of each first stage tank. Steam from the first stage tanks is transmitted to the second stage tank by a plurality of conduits extending from first stage tanks to a central manifold feeding an encircled second stage tank.
Experiments have continued over the years with apparatus constructed in accordance with prior U.S. Pat. No. 4,408,116 mentioned above. As the price of crude oil increases, more and more efforts have been undertaken to recover deposits from domestic wells. However, one common weakness in prior devices has been the inability to reliably and virtually continuously generate and deliver a high volume of pressurized, superheated steam at temperature approximating 1200 degrees F. One problem has been experienced with the electrodes used to heat internal vaporization tanks, and with other critical components. Wide temperature variations are encountered in use. Prior to energization, for example, the component temperature is that of the environment, i.e., ambient temperature. After heating commences, a temperature rise in excess of 1000 degrees F. occurs. Because of the resultant expansion of the metal components, and the various different coefficients of expansion that characterize parts of different substances, extreme stresses occur, as part dimensions increase and pressure and temperature rises.
The stress problem has caused heater tank failure in the past, necessitating frequent time consuming and expensive field repairs. For example, because of the traditional mounting techniques used for high temperature tanks, that are bathed within liquid lead during operation, tank cracking and deformation have been unavoidably frequent. These problems have been aggravated by the prior art configuration of internal electrodes used for heating the critical tanks. The proposed solution in part utilizes a new electrode configuration, combined with a flexible tank mounting configuration.
Furthermore, to reach operating temperatures approximating 1200 degrees F., the water and steam injection pathways must be carefully controlled, and energy must be conserved. While various prior art steam injection heaters have utilized piping arrangements establishing fluid flow in thermal, heat exchange relation, an adequate high temperature, superheated steam injection system must employ manifolding that is designed to conserve energy by minimizing fluid-blocking back-pressures, that are characteristic of prior art designs. Further, the entire fluid flow path must be capable of non-destructively, mechanically adapting in response to heat-induced expansions and later down-time contractions. The latter factor is particularly important with the flexible, slack-accommodating mounting of the heater tanks proposed by the instant invention, and with the chosen electrode configuration, the use of which has been enabled by said heater tank mounting arrangement.